1. Field of the Invention
The present invention relates to thermal heads mounted in thermal printers or the like. The present invention particularly relates to a thermal head which can suppress voltage drop in a common electrode and can uniformly generate heat along an array of thermal head elements formed in the vicinity of the end of a substrate.
2. Description of the Related Art
In general, a thermal recording head mounted in a thermal printer includes an array of, or a plurality of arrays of, heating elements composed of heating resistors disposed on a substrate. When these heating elements are selectively energized in response to printing information, the heat generated by the elements colors a thermal recording sheet or melts and transfers ink on an ink ribbon onto plain paper or a transparent sheet.
FIG. 15 shows a conventional thermal head. A heat-insulating layer 112 composed of a glass glaze is formed over an entire heat dissipating substrate 111 composed of an electrically insulating ceramic such as alumina. A projecting section 113 which protrudes from the heat-insulating layer 112 is formed by etching or the like in the vicinity of the end 111a of the heat dissipating substrate 111. A first common lead layer 114a with a thickness of approximately 1 .mu.m is formed on the entire heat-insulating layer 112 by a sputtering process or the like. The first common lead layer 114a is composed of a hard, heat-resistant high-melting point metal, such as chromium, having high adhesiveness to the heat-insulating layer 112. The first common lead layer 114a preferably has a large area and a large thickness of approximately 1 .mu.m to reduce resistance thereof. Furthermore, a second common lead layer 114b is deposited on the entire surface of the first common lead layer 114a by a sputtering process. The second common lead layer 114b is composed of a cermet, which is a composite material of a metal and an insulating ceramic, such as Ta--SiO.sub.2 (hereinafter, a Ta-containing cermet is referred to as a"Ta cermet").
A strip antioxidative mask layer adjacent to the projecting section 113 is formed between the end 111a of the heat dissipating substrate 111 and the projecting section 113 of the heat-insulating layer 112 and on the second common lead layer 114b. The second common lead layer 114b is heated to approximately 700.degree. C. so that the second common lead layer 114b is thermally oxidized over several thousands angstroms from the surface, except for the portion covered by the antioxidative mask layer. A first insulating interlayer 115a composed of oxide ceramic having significantly decreased defects is thereby formed.
The portion protected by the antioxidative mask layer remains as a conductive section 116. The antioxidative mask layer is removed to expose the conductive section 116 on the first insulating interlayer 115a.
A second insulating interlayer 115b composed of an insulating ceramic such as SiO.sub.2 is deposited on the first insulating interlayer 115a by a sputtering process or the like, and then a contact hole 115c is formed in the second insulating interlayer 115b so that the conductive section 116 is exposed from the second insulating interlayer 115b.
An underlying common electrode 117a composed of a high-melting point metal such as chromium is formed on the second insulating interlayer 115b so as to cover the conductive section 116. An array of strip underlying discrete electrodes 118a composed of a high-melting point metal such as chromium is formed on the second insulating interlayer 115b. These underlying discrete electrodes 118a oppose the underlying common electrode 117a at a predetermined distance above the projecting section 113.
A plurality of strip heating elements 119 composed of a Ta cermet is provided over the strip underlying discrete electrodes 118a and the underlying common electrode 117a. Thus, each heating element 119 forms a heating zone S1 between the underlying common electrode 117a and the respective underlying discrete electrode 118a.
Overlying discrete electrodes 118b composed of aluminum or copper are connected to the underlying discrete electrodes 118a through the heating elements 119. The overlying discrete electrodes 118b extend to the other terminal end of the heat dissipating substrate 111, away from the end 111a. Electrical power is supplied to each overlying discrete electrode 118b thorough the other terminal end.
An overlying common electrode 117b composed of aluminum or copper is formed on the strip heating elements 119 so as to oppose the underlying common electrode 117a. Furthermore, a protective layer 120 with a thickness of approximately 5 .mu.m is deposited over the heating elements 119, the overlying common electrode 117b, and the overlying discrete electrodes 118b other than the terminal section for an external circuit, by a sputtering process or the like. The protective layer 120 is composed of a material, such as sialon (a solid solution of a Si--Al--O--N compound), having high oxidation resistance and abrasion resistance.
These overlying discrete electrodes 118b are energized based on given printing information. A current from a overlying discrete electrodes 118b flows in the respective underlying discrete electrode 118a and the respective heating element 119, and flows in the underlying common electrode 117a, the overlying common electrode 117b, the conductive section 116, and the first and second common lead sublayers 114a and 115b toward the external circuit.
In a typical conventional thermal head including driver ICs, a glazed aluminum substrate is generally used in which a glass material is glazed on a heat dissipating substrate composed of alumina or the like. A plurality of linear heating elements is arranged in the vicinity of the end of the substrate. These heating elements are selectively energized according to recording information. The heat generated in the heating elements records dot images on thermal recording paper or plain paper by ink transfer from a thermal transfer ink ribbon provided between the thermal head and the plain paper.
FIGS. 16 and 17 are a cross-sectional view and a schematic plan view, respectively, of a main section of another conventional thermal head. A glass heat-insulating layer 202 is formed on a heat dissipating substrate 201 composed of an insulating ceramic such as glazed alumina. The heat-insulating layer 202 has a projection 202a having a trapezoidal cross-section at the end region. A first common lead layer 203a, which is composed of a high-melting point metal and has a thickness of approximately 1 .mu.m, and a second common lead layer 203b, which is composed of a cermet of a high-melting metal and SiO.sub.2 and has a thickness of approximately 1 .mu.m, are formed on the heat-insulating layer 202 including the projection 202a by a sputtering process or the like. An antioxidative conductive metal such as MoSi.sub.2 or antioxidative insulating ceramic such as SiO.sub.2 with a thickness of approximately 0.2 .mu.m is formed on the second common lead layer 203b by a sputtering process. The antioxidative material is etched to form a thermal-oxidation mask layer 204 with a predetermined pattern for providing contact holes by a photolithographic etching process.
The substrate 201 is heated to approximately 600.degree. C. to 800.degree. C. to form a first insulating interlayer 205a on the exposed region of the second common lead layer 203b which is not covered with the thermal-oxidation mask layer 204, by thermal oxidation. A second insulating interlayer 205b composed of SiO.sub.2 or the like is formed on the first insulating interlayer 205a. Such a double-layered configuration enhances reliability of interlayer insulation. A contact hole 205c is formed in the second insulating interlayer 205b at the position corresponding to the thermal-oxidation mask layer 204 by a photolithographic etching process. A substrate provided with the layered common electrode is thereby formed. An electrode material composed of a high-melting point metal such as molybdenum is deposited on the second insulating interlayer 205b by a sputtering process or the like, and an electrode pattern for an underlying common electrode 206 and underlying discrete electrodes 207 is formed by a photolithographic etching process.
A heating element layer composed of Ta--SiO.sub.2 or the like is deposited on the electrode pattern. The heating element layer is etched by a photolithographic etching process to form an array of heating elements 208 corresponding to the number of dots. Any other electrode configuration may also be employed. For example, heating elements 208 with a given pattern are previously formed, and chromium electrodes are deposited on the heating elements 208.
An aluminum or copper overlying electrode layer with a thickness of approximately 2 .mu.m is formed on the heating elements 208, for supplying electrical energy. Since the multi-layered common electrode is provided at one side of the heating elements 208, no overlying common electrode is necessary at this side. Thus, only three common terminals 209 for external connection for connecting the first and second common lead layers 203a and 203b to an external circuit are formed on three contact holes 205c provided at the two ends and in the center of the substrate 201 (see FIG. 17).
Overlying discrete electrodes 210 for independently heating the heating elements 208 are formed at the other sides of the substrate 1, and first pads 210a for connecting driver ICs 211 are formed at the ends of heating elements 208. Second pads 210b for connecting the driver ICs 211 and discrete terminals 210c for connecting the external circuit are also arranged so as to form an array including the common terminals 209 for connecting the external circuit. These terminals 209a and 210c and pads 210a and 210b are plated and connected to the driver ICs 211 and a flexible printed circuit (FPC) 213 as an external circuit by soldering or contact bonding.
A SiO.sub.2 or sialon protective layer 212 having high hardness with a thickness of approximately 5 .mu.m is formed on the heating elements 208 and the overlying discrete electrodes 210 to prevent oxidation and abrasion of these units and electrodes by a sputtering process. The protective layer 212 substantially covers the entire surface other than the terminals 209a and 210c and the pads 210a and 210b. After terminal plating, the substrate 201 is cut by a dicing process to form block thermal heads.
In a thermal printer using the conventional thermal head, the overlying discrete electrodes 210 are energized through the respective driver ICs based on the recording signals to selectively heat these heating elements 208 of the thermal head. The heated heating elements 208 transfer ink on a thermal transfer ink ribbon (not shown in the drawing) onto a recording sheet, or colors a thermal recording sheet on a platen (not shown in the drawing), to form a recorded image.
In such a conventional thermal head, the chromium first common lead layer 114a must have a large thickness or a large area in order to reduce the resistance and thus to reduce common drop of voltage in the common electrode layer which would leads to deterioration of the quality of the printed image.
When the thickness of the first common lead layer 114a composed of a high-melting point metal such as chromium is large, for example, 1 .mu.m, the layer formed by a sputtering process inevitably has large residual stress in proportion to the thickness due to large tensile stress. Thus, the interfacial bonding strength between the first common lead layer 114a and the heat-insulating layer 112 decreases by high-temperature thermal oxidation treatment for forming the first insulating interlayer 115a in the subsequent step, by thermal impact during a high-temperature high-vacuum treatment performed for stabilizing the heating elements 119, and by mechanical impact in the subsequent steps. As a result, the quality and the production yield of the thermal head products decrease.
When the common lead layers 114a and 114b are formed above substantially the entire heat dissipating substrate 111, the probability of insufficient insulation between the common lead layers 114a and 114b and the overlying discrete electrodes 18b due to defects in the insulating interlayers 115a and 115b increases in proportion to the area of the common lead layers 114a and 114b, resulting in decrease in the quality and the production yield of the thermal head products.
Since a chromium first common lead layer 114a having a large thickness of 1 .mu.m and high thermal conductivity is present below the heating zone S1, the first common lead layer 114a dissipates heat generated in the heating zone S1. Thus, the heating zone S1 cannot be rapidly heated, and the quality of the printed image deteriorates due to decreased thermal printing efficiency.
Since the first common lead layer 114a and the second common lead layer 114b are formed above the entire heat dissipating substrate 111, the first and second common lead sublayers 114a and 114b are exposed at the end 111a of the heat dissipating substrate 111. As a result, leakage and short-circuiting to external units will occur.
As described above, three contact sections of the common terminals 209 for connecting the external circuit and the common lead layers 203a and 203b are provided at both ends of the substrate and in the center of the array of the heating elements 208. Thus, the common lead layers 203a and 203b between the common terminals 209 for connecting the external circuit and the heating elements 208 inevitably have a large length L, and the current path lengths to the heating elements 208 are different from each other.
On the other hand, the common lead layers 203a and 203b are composed of a high-melting point metal having larger resistivity than that of aluminum or copper. Since the distances between the heating elements 208 and the common terminals 209 for connecting the external circuit are different from each other, the resistances of the common electrode to the heating elements 208 are also different from each other. Thus, the array of the heating elements does not have a uniform temperature distribution which is essential for uniform recording density. When the thickness of the common lead layers is increased in order to solve such a problem, the hard film composed of a high-melting point metal has large tensile strength causing production defects such as interlayer separation, resulting in decrease in quality and yield.
When the common lead layers contain a thick metal layer, heat generated in the heating elements readily dissipates through the metal layer. Thus, the thermal head has low thermal efficiency.